1. Field of the Invention
This invention relates to an apparatus for injection molding of parts from foam plastics material and/or from homogeneous plastics material. When foam plastics material is required, an inert gas is injected into the melt zone of the screw and barrel assembly under high pressure and the separate bubbles formed are mixed with the molten plastics material as a result of the rotation of the screw member in the barrel member to form the foam plastics material.
2. Description of the Prior Art
Industry has recognized for many years the advantages of utilizing parts made from foam plastics material as well as from homogeneous plastics material. A variety of methods and machines for forming and molding plastics material, with or without a foaming agent, have been utilized. The advantages of foam plastics material include strength, light weight, economy and the surface characteristics of the foam plastic parts. In the past, when a foam plastic part is used with a homogeneous plastic part, it has been the common practice to manufacture the foam plastic parts separately from the homogeneous plastic parts through separate injection devices and apparatuses, even using separate presses to mold the homogeneous parts and the foam plastic parts. This is expensive and often results in an over or an under count of one part relative to the other.
It has also been the common practice to operate the screw and barrel member of the extruder intermittently thus creating technical and maintenance problems in the manner in which the gas is introduced and mixed with the molten plastics material in the extruder. In certain instances where gas emitting fluids or gas emitted solid materials are premixed with plastic pellets, such operations are expensive not only due to the cost of the gas emitting materials but also due to the cost of the premixing procedures. Finally, the manner in which the gas is introduced into the molten plastics material has in the past resulted in many maintenance problems which can be corrected or minimized if the screw member is operated continuously as proposed herein.
In the prior art when the extruder or screw member is operated intermittently and stops it behaves like a coil spring in that it tends to rotate backward and the plastics material wants to backflow. As this occurs, the pressure in the extruder drops with the result that the inert gas forms a very large bubble in the main passageway so that when the extruder or screw member is restarted, it is necessary to break the bubble and whip it into the plastic material. This is difficult to achieve on an ongoing basis. Also it has been found that the molten plastics material will tend to clog the conduits or passages in which the inert gas is delivered to the main passageway of the screw and barrel members upon intermittent operation thereof. Such problems tend to reduce the quality of the foam plastics parts and the quantity of parts manufactured in a given time period. It further adds to the costs of manufacture and to the maintenance problems of the manufacturer.